Sand cores are used to form the internal cavities of a finished casting. When sand cores are placed in a mold and molten metal is introduced into the mold, a rapid thermal expansion of the sand in the sand cores takes place. As a result of the rapid thermal expansion of the sand in the sand core, the sand core cracks, and the molten metal runs into the cracks in the core, creating a fin projecting from the casting surface (in foundry terms, a “vein”) as the molten metal solidifies. These veining defects, caused by uncontrolled core sand thermal expansion, are most often controlled by anti-veining or expansion control agents, which are mixed uniformly with the sand and core sand binders prior to the formation of the sand cores themselves. Anti-veining or expansion control agents change the thermal coefficient of expansion of the sand core to control its cracking and reduce or eliminate the formation of veins.
For years, iron oxides were used in foundries to improve sand cores and the qualities of castings. Iron oxides proved to be advantageous in sand cores by reducing the formation of thermal expansion defects such as veining. Iron oxides in use include red iron oxide (Fe2O3), also known as hematite, black iron oxide (Fe3O4), known as magnetite, and yellow ochre. The most common methods of employing such iron oxides are by addition of approximately 1% to 3% by weight to the core sand during mixing. The mechanism by which iron oxides improve the surface finish is not known. One theory is that the iron oxides increase the plasticity of the sand core during casting by formation of sand grain interfaces which deform, or give, without fracturing, thereby preventing cracks in the core which can form veins in the casting.
U.S. Pat. No. 4,735,973 discloses an additive for the foundry sands used to produce cores and molds which improves the quality of the castings by reducing thermal expansion and gas defects, thereby reducing the veins formed in a casting. The disclosed additive comprises a composition containing from about 15% to about 95% titanium dioxide (TiO2), including a preferable additive comprising about 2% to about 38% silicon dioxide (SiO2), about 5% to about 40% ferric oxide (Fe2O3), about 15% to about 95% titanium dioxide (TiO2), and about 2% to about 45% aluminum oxide (Al2O3). The resulting sand cores are described as comprising about 80% to about 98% of core sand aggregates selected from a group consisting of silica sand, zircon sand, olivine sand, chromite sand, lake sand, bank sand, fused silica, and mixtures thereof, about 0.5% to about 10% of a core sand binder, and about 0.5% to about 5% of an additive composition containing from about 15% to about 95% titanium dioxide (TiO2). The use of such additives in sand cores is described as reducing the casting defects associated with the use of plastic bonded and other core binder systems, increasing the strength of the resulting bonded core sand, and allowing a reduction in the amount of plastic binder required.
U.S. Pat. No. 5,911,269 discloses a method of making silica sand cores utilizing lithium-containing materials that provide a source of lithium oxide (Li2O) to improve the quality of castings by reducing sand core thermal expansion and the veins resulting therefrom in metal castings. The disclosed method of making sand cores comprises the steps of preparing an aggregate of sand core and a resin binder, and mixing into the aggregate a lithium-containing additive selected from a group consisting of .α.-spodumene, amblygonite, montebrasite, petalite, lepidolite, zinnwaldite, eucryptite and lithium carbonate, in the amount to provide from about 0.001% to about 2% of lithium oxide. The use of such a method and lithium oxide-containing additives is described as reducing the casting defects associated with thermal expansion of silica, including the formation of veins in the cavity and improving the surface finish of the castings.
Examples of commercially available anti-veining additives include VEINSEAL 14000 (IGC Technologies Inc., Milwaukee, Wis.). VEINSEAL 14000 comprises 60-70 wt. % SiO2, 10-20 wt. % Fe3O4, 15-25 wt. % Al2O3, 10-25 wt. % TiO2 and 2-5 wt. % Li2O. Typical loadings of VEINSEAL in a foundry mold is about 1-5 wt. % on the total weight of the foundry aggregate. Unfortunately, the costs, even at this low dosage are relatively high. Other commercially available anti-veining agents include VEINSEAL 15000 from IGC Technologies, and VEINO Ashland Inc.